ENH: respond to externalCoupled lock file contents

- waitForSlave now return a Time::stopAtControls enumeration:

    unknown:     when lockfile has no specially recognized content.
    endTime:     when lockfile contains "status=done"
    writeNow:    when lockfile contains "action=writeNow"
    nextWrite:   when lockfile contains "action=nextWrite"
    noWriteNow:  when lockfile contains "action=noWriteNow"

These values can be used by the caller to terminate the master
(OpenFOAM) as desired in response to information placed there by the
slave process.